1. Field of Invention
The present invention is related to processing of dielectric or semiconductor materials. More particularly, the invention is related to releasing a dielectric or semiconductor workpiece from an electrostatic chuck.
2. Description of Related Art
During the process of wafer processing or glass processing, an electrostatic chuck (ESC) is used to clamp the semiconductor or dielectric workpiece to a metal holder. The ESC operates like a capacitor in that a static charge is built up between the workpiece and the metal holder to clamp or secure the workpiece on the metal holder. The process of semiconductor or dielectric processing deposits a substantial amount of power on the workpiece. In order to cool the workpiece during processing, a heat transfer medium is needed to transfer heat from the workpiece to a heat sink. The heat sink is the metal holder which has been water cooled. The heat transfer medium is a gas such as helium which is capable of transferring heat from the workpiece to the water-cooled metal holder. The ESC is the preferred clamping device to hold the workpiece against the metal holder.
Upon completion of the processing, the workpiece must be removed from the metal holder. A residual sticking force remains between the ESC and the workpiece after the clamping force has been turned off. The process of removing the workpiece due to this sticking force is referred to as “dechucking” or declamping the workpiece. The dechucking process is complicated by the variation in dechucking values for each individual ESC. Therefore, the dechucking values used for one ESC do not necessarily work for another ESC, even if each ESC is made by the same manufacturer using the same materials. Additionally, a single ESC has electrical properties that depend on temperature, so the dechucking values at one temperature do not necessarily work for the same ESC at a different temperature.
Prior art systems and methods for dechucking teach a variety of different dechucking methods. By way of example, Watanabe et al. in U.S. Pat. No. 5,117,121 teaches a dechucking apparatus that applies a reverse polarity voltage for a period of time. The patent teaches different dechucking voltages and dechucking time that may be stored in a computer for use with different values of the clamping voltage and for different durations of the clamping voltage. However, the patent fails to teach, inter alia, a method for determining the optimum dechucking parameters. Additionally, the patent fails to teach a system and method that accounts for the variability in the electrical properties for each ESC. Further still, the patent fails to address how the changes in temperature affect the dechucking process.
In U.S. Pat. No. 5,459,632, U.S. Pat. No. 5,612,850, and U.S. Pat. No. 5,491,603 Birang et al. describes a dechucking method in which an optimum dechucking value is determined that allows the workpiece to be removed from the chuck with much less force. Birang teaches an automatic method of establishing the dechucking voltage by connecting the chucking voltage supply between the chuck electrode and the semiconductor wafer while the wafer is some distance above the chuck, and then measuring the surge of current flow from the chucking voltage supply when the wafer is lowered on to the upper dielectric of the chuck. Furthermore, Birang teaches a method that determines the optimum dechucking voltage by reducing the applied potential on the ESC electrode from the relatively high chucking voltage downwardly in small steps while monitoring the leak rate of the helium beneath the wafer. However, Birang fails to teach an efficient system and method for calculating the dechucking voltage and time at various temperatures for a batch of ESCs. Additionally, Birang fails to teach the monitoring of the current when the workpiece is lifted from the chuck. Further still, Birang fails to teach a method to measure the chuck resistance.
In U.S. Pat. No. 5,793,192, Kubly et al. teaches the use of dechucking voltage for a dechucking period and provides a method for choosing the optimum dechucking values. The dechucking values of voltage and time are calculated by supplying various clamping voltages, and measurements are taken of the time it takes for the heat transfer gas to pop the wafer off the chuck. This method of calculating voltages and times is inefficient because each chuck must be analyzed individually at the various clamping voltages. Additionally, this method does not take into consideration temperature effects.
In U.S. Pat. No. 5,818,682, Loo teaches a method and an apparatus that predicts an optimal period over which a dechucking voltage is applied to an ESC to achieve dechucking of a workpiece. Loo teaches a dechucking period and dechucking voltage that is related to the chucking period, chucking voltage and a constant. Loo's method and apparatus fails to use temperature, chuck current, and chuck resistance for determining dechucking parameters of reverse voltage and time.
In U.S. Pat. No. 6,125,025, which is hereby incorporated by reference, Howald et al. teaches at least one method of dechucking. The method includes the process steps of reducing the voltage applied to the dielectric workpiece without reversing the polarity of the voltage via the electrode and insulator while workpiece processing proceeds. Another of the dechucking steps includes reversing the polarity of the DC voltage applied to the electrode upon completion of the workpiece processing. However, the method fails to teach the use of temperature and chuck resistance for determining dechucking parameters of reverse voltage and time.
Therefore, it would be beneficial to provide a system and method for dechucking that is simple to implement.
Additionally, it would be beneficial to provide a system and method for dechucking that takes into consideration the temperature effects on the ESC.
Further still, it would be beneficial to provide a system and method for dechucking that takes into consideration the electrical properties for each ESC.
Furthermore, it would be beneficial to provide a system and method for dechucking that is dependent on the unique properties associated with each ESC.
Further still, it would be beneficial to provide a system and method for dechucking that can be implemented for a batch of ESCs.
Further still, it would be beneficial to provide a system and method for dechucking that is not dependent on the length of time for which the chucking voltage is applied.
Further still, it would be beneficial to provide a system and method that can reliably model the ESC dechucking values.
Further still, it would be beneficial to provide a system and method for dechucking that can be implemented by programming an etch tool to perform the dechucking operations rather than present day methods that rely on the tool operator.